1. Field of the Invention
The present invention relates to filtration systems. Filtration systems have conventionally been used for either (1) removal of particulate matters from fluid suspensions to result in clear, non-turbid fluids or, (2) removal and discarding of part of the soluble and fluid fraction for the purpose of concentrating the desirable particulate matters. To achieve the first purpose, the filter in the system is used to trap the particulate matters by virtue of the effective pore sizes being smaller than the particulate matters, while allowing the soluble fraction to go through the filter pores and collected for subsequent use. To achieve the second purpose, the ideal filter will allow the soluble fraction to go through the filter pores with only minimal entrapment of the particulate matters which are then returned to collection containers as the "retentate" fraction for subsequent use. In both procedures, clogging of the filter remains a major problem.
2. Description of the Prior Art
The clogging of filter pores is a major problem with prior art filtration and dialysis devices. Clogging of the filter pores quickly reduces the efficiency of the filtration system. As the number of unclogged pores diminishes, filtration rate decreases. Since flow rate is equal to pressure gradient divided by resistance, as more and more filter pores are clogged (increasing resistance), a progressively larger pressure gradient is needed to maintain adequate flow rates. Even then, when enough of the filter pores become clogged, flow rate will become for all practical purposes, zero. At that point, particulate matters can no longer be removed from fluid suspensions. In addition, if the purpose is to concentrate suspended particulate matters, clogging of filters will decrease the final yield of the particulate matters and may in fact decrease the concentration of such matters in the retentate.
To minimize the problem of clogging, various approaches have been designed, as reflected in different filtration systems on the market. One approach incorporates designs for vigorous stirring of the suspension physically above, or prior to interaction of the suspension with the filter surface. Examples include the Stirred Cells Series of Amicon Division, W. R. Grace & Co. However, such systems are ineffective because the distance between the stirring mechanism and the filter membrane (typically in millimeters) are several orders of magnitude larger than the diameter of the particles (typically in microns). Once the particulate matters are trapped within the filter pores, with constant positive filtration pressure pressing them against the filter membrane, agitation at a far distance (relative to the size of the particulate matters) will not effectively dislodge them. Moreover, high shearing forces generated by vigorous stirring may cause foaming and denaturation of the particulate matters.
Another approach involves the concept of tangential flow as exemplified by Millipore's Minitan system. Instead of applying pressure perpendicular to the surface of the filter, the suspension is pushed forward by positive pressure from a pump system so that it travels in a direction tangential to the filter surface. In theory, this design allows the particulate matters to travel in a direction tangential to the filter surface while the soluble phase goes through the filter pores in a direction perpendicular to the filter surface. In practice, substantial clogging still occurs. The reason is that the particulate matters are carried by the soluble phase of the suspension and will travel in the direction of the immediate fluid surrounding a given particle. Any time a fraction of the soluble phase goes through the filter pores (in a direction perpendicular to the filter surface), a proportional amount of particulate matters will travel with it in the same direction. Regardless of the direction of flow of the rest of the suspension bulk (which may travel in a direction tangential to the filter surface), the fraction that goes through the pores will clog up the pores. With this understanding, it becomes clear that tangential flow filter systems are only different ways of recirculating the bulk of the suspension before its interaction with the filter pores. This design does not substantially alter the clogging potentials of particulate matters at the level of the filter pores because the particulate matters are again pressed onto the pores by the positive pressure used to circulate the bulk of the suspension.
Since both the stirred cell design and the tangential flow systems use positive pressure to circulate the suspension, they both result in trapping of particulate matters within the matrix of the filter membrane. For this reason, these systems are not suitable for the purpose of concentrating particular matters. There exists a need for a novel design where: (1) the filter membrane will not be clogged, and (2) should unexpected change in the filtration condition lead to some clogging, the obstructed pores will become unclogged again. Such a device will allow efficient concentration of valuable particulate matters. In addition, because of the increased life span of the filter membrane, it also allows cost-efficient collection of the soluble phase of the suspension, if the soluble phase is the desirable fraction from the suspension.